The invention relates to transdermal alcohol sensors (TAS), and more particularly to a potential-controlled and diffusion-controlled solid electrolyte sensor for continuous measurement of very low levels of transdermal alcohol.
The reliable and valid measurement of alcohol consumption or abstinence by passive, noninvasive means, over long time periods is important for monitoring individuals in research studies, monitoring those undergoing substance abuse treatment, and for forensic monitoring in special populations. Methods traditionally used for determining alcohol consumption include verbal report measures, biochemical markers, and direct measurement of urine, blood, saliva or sweat alcohol levels. The ideal method for determining alcohol consumption would be inexpensive, highly specific and sensitive for ethanol, applicable across a wide range of users, usable over a range of time periods, and accepted by both the testers and the tested.
One promising method, the "sweat patch," of monitoring alcohol use has used the detection of ethanol transdermally. Measurable quantities of ingested ethanol are excreted through the human skin, by exocrine sweat glands, and by diffusion across the skin. Transdermal ethanol levels generally reflect blood levels.
The alcohol dosimeter or "sweat patch" is a wearable, noninvasive device which is reported to accumulate ethanol in an absorbent medium to assess cumulative ethanol consumption over a 7- to 10-day period. However, field trials of the sweat patch identified problems limiting its clinical utility. These include problems with ethanol storage and losses due to evaporation, back-diffusion and bacterial metabolism.
Electrochemical detection of ethanol has been used for many years in sensor cells which oxidize ethanol and produce currents proportional to the ethanol concentration. Such cells are used in commercially available portable breathalysers (e.g., Alco-Sensor III by Intoximeters, Inc., St. Louis, Mo.). The readings are well correlated with blood alcohol concentration (BAC). Application of this device to periodically measure alcohol accumulated in sweat-patch experiments, as described above, does not supply continuous alcohol consumption data.
Methods and apparatus for determining alcohol concentration in blood using electrochemical breath alcohol measurements are extensive and examples of these are described in U.S. Pat. Nos. 3,824,167; 3,824,168; 3,838,971; 3,909,386; 3,966,579; 4,314,564; 4,317,453; 4,499,423; Re. 31,915; 4,925,544; 4,976,135; 5,336,390. Most of these processes use liquid electrolytes or solid gelled liquid electrolytes containing an ionic salt or acid. A solid polymer electrolyte electrochemical cell, containing no liquid electrolyte, has also been used for the detection of alcohol as described in U.S. Pat. No. 4,820,386.
The typical breath alcohol sensing devices, such as the devices sold by Intoximeter (St. Louis, Mo.), measure alcohol concentrations present in a gaseous breath sample by measuring the magnitude of the short circuit current passing through the external circuit between the anode and cathode of a fuel cell. The potential of the anode (or sensing electrode) prior to ethanol exposure is determined by the air (O.sub.2) cathode and will change depending on the oxidation current due to alcohol consumption on the anode. The sensing anode potential is not fixed or controlled. It drifts below the Pt/air (O.sub.2), 1.06 V rest potential (vs. a Normal Hydrogen Electrode, N.H.E.) when current passes from the sensing anode to the air cathode. A potentiostatic method of controlling the sensing electrode at a fixed potential above the potential of a Pt/air (O.sub.2) reference electrode located on the same piece of solid polymer electrolyte membrane is described in U.S. Pat. No. 4,123,700. Several of the breath alcohol sensors described above utilize 1) methods for mechanically introducing, periodically, a fixed amount of breath alcohol into sample chambers or 2) detection methods to indicate breath alcohol is validly being sampled (e.g., water vapor detectors, pressure switch). The detection and sampling processes are not applicable to the accurate and reproducible continuous monitoring/recording of transdermal alcohol vapor.
No single method currently exists to easily and reliably quantify patterns of alcohol consumption or abstinence over a period of days, weeks or months. Existing commercial electrochemical sensors are not suitable for continuous and reliable longterm measurement of alcohol. They are not sufficiently sensitive, selective, reproducible, or passive (i.e., they require active subject participation).
Therefore it is a principal objective of this invention to provide an electrochemical sensor based on a solid polymer electrolyte in the proton exchange form integrated with a recording device that is wearable and provides 1) continuous rather than episodic monitoring of blood alcohol levels by passive tracking of very low concentrations of transdermal alcohol, and 2) selective, accurate, semiquantitative and temporal tracking of ethanol consumption or abstinence over extended periods.
Still another object of this invention is to provide a process and method for continuously monitoring transdermal ethanol by an electrochemical solid polymer electrolyte sensor, which includes the sensor cell, diffusion-control membrane and potential-control circuitry and a data acquisition-recording device unitized into a single instrument.
Still another object of this invention is to provide a solid polymer electrolyte sensor and process that provides potential and diffusion control to the sensor cell, leading to rapid electrochemical oxidation of ethanol and establishment of a steadystate flux of ethanol from skin to sensing electrode surface.
Still another object of this invention is to have an integral diffusion-control membrane over the sensing electrode that is in series with the human stratum corneum and 1) controls ethanol and water flux from the skin contact area to the solid polymer electrolyte sensor cell and normalizes ethanol permeation differences due to the skin properties and 2) helps manage humidification of the sensor cell solid polymer electrolyte membrane.
Still another object of this invention is to provide a solid polymer electrolyte sensor that concurrently, with transdermal alcohol measurement, also measures skin properties such as temperature and ionic conductivity to determine continuous contact with the skin.